Eldest children score higher than their younger siblings in IQ tests because of increase mental stimulation from their parents. 

New research from the University of Edinburgh found first-born children are given an increased advantage because their parents take more time to stimulate their brain and take fewer risks.

Researchers found IQ scores were higher among first born children by age one, and the gap widened as they grew older.

Although emotional support remained largely the same, parents offered less mental stimulation to younger siblings also took part in fewer activities such as reading with the child, crafts and playing musical instruments.

Analysis of 5,000 children was done by the University of Edinburgh, Analysis Group and the University of Sydney. Every child was assessed every two years.

Research

The tests included reading recognition, such as matching letters, naming names and reading single words aloud and picture vocabulary assessments.

Information was also collected on environmental factors such as family background and economic conditions.

Researchers applied statistical methods to economic data to analyse how the parental behaviour of the child was related to their test scores.

Parental behaviour was compared to assessments, including pre-birth behaviour such as smoking and drinking activity during pregnancy, and post-birth behaviour such as mental stimulation and emotional support.

Researchers said the findings could help to explain the so-called birth order effect when children born earlier in a family enjoy better wages and more education in later life.

Dr Ana Nuevo-Chiquero, of the University of Edinburgh’s School Economics, said: "Our results suggests that broad shifts in parental behaviour are a plausible explanation for the observed birth order differences in education and labour market outcomes."

The study is published in the Journal of Human Resources.